The present invention relates to superabsorbent gels containing a poly(vinylamine), or a salt thereof, and to an improved method of manufacturing a poly(vinylamine). The superabsorbent gels comprise a basic superabsorbent polymer, and preferably a poly(vinylamine), in an amount of about 20% to about 40% by weight of superabsorbent polymers, admixed with an acidic superabsorbent polymer, like a polyacrylic acid, or comprise a salt of a poly(vinylamine).
Water-absorbing resins are widely used in sanitary goods, hygienic goods, wiping cloths, water-retaining agents, dehydrating agents, sludge coagulants, disposable towels and bath mats, disposable door mats, thickening agents, disposable litter mats for pets, condensation-preventing agents, and release control agents for various chemicals. Water-absorbing resins are available in a variety of chemical forms, including substituted and unsubstituted natural and synthetic polymers, such as hydrolysis products of starch acrylonitrile graft polymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidines, and polyacrylonitriles.
Such water-absorbing resins are termed xe2x80x9csuperabsorbent polymers,xe2x80x9d or SAPs, and typically are lightly crosslinked hydrophilic polymers. SAPs are generally discussed in Goldman et al. U.S. Pat. No. 5,669,894. SAPs can differ in their chemical identity, but all SAPs are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. For example, SAPs can absorb one hundred times their own weight, or more, of distilled water. The ability to absorb aqueous fluids under a confining pressure is an important requirement for an SAP used in a hygienic article, like a diaper.
The dramatic swelling and absorbent properties of SAPs are attributed to (a) electrostatic repulsion between the charges along the polymer chains, and (b) osmotic pressure of the counter ions. It is known, however, that these absorption properties are drastically reduced in solutions containing electrolytes, such as saline, urine, and blood. The polymers do not function as effective SAPs in the presence of such physiologic fluids.
The decreased absorbency of electrolyte-containing liquids is illustrated by the absorption properties of a typical, commercially available SAP, i.e., sodium polyacrylate, in deionized water and in 0.9% by weight sodium chloride (NaCl) solution. The sodium polyacrylate can absorb 146.2 grams (g) of deionized water per gram of SAP (g/g) at 0 psi, 103.8 g of deionized water per gram of polymer at 0.28 psi, and 34.3 g of deionized water per gram of polymer of 0.7 psi. In contrast, the same sodium polyacrylate is capable of absorbing only 43.5 g, 29.7 g, and 24.8 g of 0.9% aqueous NaCl at 0 psi, 0.28 psi, and 0.7 psi, respectively. The absorption capacity of SAPs for body fluids, like urine or menses, therefore, is dramatically lower than for deionized water because such fluids contain electrolytes. This dramatic decrease in absorption is termed xe2x80x9csalt poisoning.xe2x80x9d
The salt poisoning effect has been explained as follows. Water-absorption and water-retention characteristics of SAPs are attributed to the presence of ionizable functional groups in the polymer structure. The ionizable groups typically are carboxyl groups, a high proportion of which are in the salt form when the polymer is dry, and which undergo dissociation and salvation upon contact with water. In the dissociated state, the polymer chain contains a plurality of functional groups having the same electric charge and, thus, repel one another. This electronic repulsion leads to expansion of the polymer structure, which, in turn, permits further absorption of water molecules. Polymer expansion, however, is limited by the crosslinks in the polymer structure, which are present in a sufficient number to prevent solubilization of the polymer.
It is theorized that the presence of a significant concentration of electrolytes interferes with dissociation of the ionizable functional groups, and leads to the xe2x80x9csalt poisoningxe2x80x9d effect. Dissolved ions, such as sodium and chloride ions, therefore, have two effects on SAP gels. The ions screen the polymer charges and the ions eliminate the osmotic imbalance due to the presence of counter ions inside and outside of the gel. The dissolved ions, therefore, effectively convert an ionic gel into a nonionic gel, and swelling properties are lost.
The most commonly used SAP for absorbing electrolyte-containing liquids, like urine, is neutralized polyacrylic acid, i.e., containing at least 50%, and up to 100%, neutralized carboxyl groups. Neutralized polyacrylic acid, however, is susceptible to salt poisoning. Therefore, to provide an SAP that is less susceptible to salt poisoning, either an SAP different from neutralized polyacrylic acid must be developed, or the neutralized polyacrylic acid must be modified or treated to at least partially overcome the salt poisoning effect.
Prior investigators have attempted to counteract the salt poisoning effect and thereby improve the performance of SAPs with respect to absorbing electrolyte-containing liquids, such as menses and urine. For example, Tanaka et al. U.S. Pat. No. 5,274,018 discloses an SAP composition comprising a swellable hydrophilic polymer, like polyacrylic acid, and an amount of an ionizable surfactant sufficient to form at least a monolayer of surfactant on the polymer. In another embodiment, a cationic gel, like a gel containing quaternized ammonium groups and in the hydroxide (i.e., OH) form, is used with an anionic gel (i.e., a polyacrylic acid) to remove electrolytes from the solution by ion exchange.
Wong U.S. Pat. No. 4,818,598 discloses admixing a fibrous anion exchange material, like DEAE cellulose, and a hydrogel, like a polyacrylate, to improve absorption properties. WO 96/17681 discloses admixing an anionic SAP, like polyacrylic acid, with a polysaccharide-based cationic SAP to overcome the salt poisoning effect. Similarly, WO 96/15163 discloses admixing a cationic SAP having at least 20% of the functional groups in a basic (i.e., OH) form with a cationic exchanges resin, i.e., a nonswelling ion exchange resin, having at least 50% of the functional groups in the acid form. WO 96/15180 discloses an absorbent material comprising an anionic SAP, e.g., a polyacrylic acid and an anion exchange resin, i.e., a nonswelling ion exchange resin.
These references disclose combinations that attempt to overcome the salt poisoning effect. It would be desirable, however, to provide an SAP that exhibits exceptional absorbency and retention, like a sodium polyacrylate, and, therefore, can be used alone as an SAP. It also would be desirable to admix such an SAP with polyacrylic acid, or another acid-containing SAP, to overcome the salt poisoning effect.
The present invention is directed to poly(vinylamine)-based superabsorbent gels. A poly(vinylamine) polymer can be used in conjunction with an acidic water-absorbing resin, like polyacrylic acid, to help overcome the salt poisoning effect, or a salt of a poly(vinylamine) polymer can be used alone as an SAP. The poly(vinylamine) polymer also can be used, alone, as an SAP to absorb and retain acidic media. More particularly, the poly(vinylamine) used as an SAP, or as a component of an SAP, is lightly crosslinked and, in preferred embodiments, is surface treated to improve absorption properties.
Accordingly, one aspect of the present invention is to provide an improved method of manufacturing a poly(vinylamine) comprising vinylamine monomer units, and which can be crosslinked using a suitable polyfunctional vinyl monomer. The present method substantially reduces the amount of residual N-vinylamide monomer in the poly(N-vinylamide) precursor of the poly(vinylamine), and, therefore, eliminates the stringent purification procedures, or reduces the long polymerization reaction times, previously used to overcome the problem of residual monomer content. Consequently, the present improved process reduces process time and production costs.
Another aspect of the present invention is to provide an SAP having absorbency and retention properties comparable to a conventional SAP, like sodium polyacrylate. A present SAP is produced by neutralizing a poly(vinylamine) with a sufficient amount of acid, like hydrochloric acid, such that at least about 10%, i.e., about 10% to 100%, of the amine-functional groups are neutralized. The resulting poly(vinylamine) salt is an excellent SAP for absorbing aqueous media.
In accordance with another important aspect of the present invention, a lightly crosslinked poly(vinylamine), alone and unneutralized, can be used to absorb and retain acidic aqueous media. The acidic aqueous media converts the low-absorbing poly(vinylamine) to a highly absorbing poly(vinylamine) salt, i.e., converts the polymer to an SAP, during absorption. A poly(vinylamine), therefore, is an excellent resin for cleaning acid spills and the remediation of acidic species.
Yet another aspect of the present invention is to provide an improved SAP that overcomes the salt poisoning effect of electrolytes. In particular, the improved SAP material contains a mixture of an acidic swellable resin, like polyacrylic acid, and about 20% to about 40%, by weight, of a poly(vinylamine), based on the total weight of the acidic resin and the poly(vinylamine).
Another aspect of the present invention is to provide an improved SAP material having improved absorption and retention properties compared to a conventional SAP, such as sodium polyacrylate. The present SAP material contains a low weight percentage of basic resin, e.g., about 20% to about 40% based on the total weight of acidic and basic resins, yet performs as well as an SAP material containing a significantly higher weight percent of basic resin. A present SAP material, therefore, provides economies by utilizing less of an expensive basic resin, while exhibiting excellent absorption and retention properties.
Yet another important feature of the present invention is to provide an SAP material containing about 60% to about 80%, by weight, of a weak acidic water-absorbing resin and about 20% to about 40%, by weight, of a weak basic water-absorbing resin, based on the total weight of the acidic and basic resin.
An example of a weak acidic resin is polyacrylic acid having 0% to 25% neutralized carboxylic acid groups (i.e., DN=0 to DN=25). Examples of weak basic water-absorbing resins are a poly(vinylamine), a poly(allylamine), and a polyethylenimine.
Still another aspect of the present invention is to provide articles of manufacture, like diapers and catamenial devices, having a core comprising an SAP material of the present invention. Other articles that can contain an SAP material of the present invention include adult incontinence products, and devices for absorbing saline and other ion-containing fluids.
These and other aspects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments.